Elderly nuns probably go out for doctor visits. It’s possible one was infected that way, and infected others, and that they were infected before the vaccination.
It is also possible that there were 26 false positive COVID tests, and the vaccine sickened the nuns. Not likely that many of them, though, unless the vaccine was tainted.
Sure, but ask yourself, how probable?
Don’t you think a nun or nuns would be honest about that?
What do they have to gain by omitting facts that might explain how they got the virus?